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LEACHING AND DISSOLVING                              351
Special Percolation Operations.—One type of percolation apparatus used largely in the cream-of-tartar industry is a semi-cylindrical perforated sheet-iron basket placed in a tank equipped with paddles. The crude argol is placed in this basket and the tank filled with water so that it covers the contents of the basket and the solution rotated by the paddles. As far as leaching is concerned it gives an excellent recovery and is a decided attempt at positive selective motion of the liquid through the solid but the power consumption is much higher than other types of leaching apparatus. It also has not the advantage of ordinary percolation over dissolution by agitation, in that the liquors are not clear after the agitation and they must be clarified in some way and furthermore it is difficult to apply the counter-current principle in this type.
In dissolving electrolytic sodium hydroxide as shipped in drums ordinarily some holes are punched in the drum and it is hung in the upper part of the tank of water. The dissolution will be fairly rapid due to the fact that the concentrated liquors being heavier than water sink, thus causing a circulation. This method is ordinarily slow and unless the material goes into solution very rapidly would not be applicable.
Leaching by Agitation.—One of the most common means of leaching is to agitate the material in the solute till the dissolution is complete, following this operation by a separation of the solids from the solution. There are two main types of apparatus for leaching by agitation. The first keeps the particles and solution in motion relative to each other and second merely holds the particles in suspension. (In quoting power consumption of machines the basis used in comparison is the power required to get equal extractions from equal tonnages of the same material.)
The paddle agitator (without baffles) consists merely of arms placed on a vertical shaft inserted through the top of a tank. It is rotated by any suitable motive power (this type is very seldom used without baffles but in order to bring out the faults of this class better we will discuss it briefly).
The only result of such an apparatus is to set the entire solution to rotating with the paddles. Fairly heavy particles will settle out from the solution very readily and after a very short time the entire system will come to equilibrium, the solution and solids and paddles all revolving at approximately the same speed and the agitation ceases, becoming only that due to retardation on sides and bottom of tank. The natural improvement is to place along the sides of the tank and between the paddles some stationary horizontal baffles.
The Baffle Paddle Agitator.—We now have a very violent agitation if sufficient baffles are used and the speed is about 10 to 15 r.p.m. The power consumption is 8 to 12 hp. for tanks 20 by 10 ft. This is due to the fact that here any relative motion of the particles through the solution is incidental to the setting up of violent eddy currents in the system. We are setting both the solids and solution in motion in the same direction so far as the mechanical action of the paddles is concerned, in direct violation of the principles already laid down. Furthermore in the space between the paddles there is no agitation and the power is consumed in that cycle in merely setting the solution in motion as in the ordinary paddle agitator though to be sure it does serve to bring the solution in those intervals to the agitating zone around the baffles but any point in the tank where the solution is not being agitated is going to retard the speed of dissolution due to the fact that it offers an opportunity for the heavier particles to settle out to the bottom